Ores are today ground in the immediate vicinity of their extraction site and pressed into pellets so they can be transported as economically as possible and optimally preprocessed especially for the smelting process. The grinding process is extremely energy-intensive, with ore extraction and (pre)processing now accounting for approximately 1.4% of global annual energy demand. Preprocessing ores as energy-efficiently as possible is accordingly highly significant in terms of protecting the climate and using resources sparingly.
With typical electric power ratings for driving mills in the range of approximately 20 MW it is very important to control the grinding process such that on the one hand the ground material's properties will conform to the specifications stipulated for pelleting. On the other hand, for energy efficiency reasons the grinding process must not last longer than is necessary to meet said specifications. That requires a sensor system which on the one hand will make a meaningful measured variable available for assessing the fineness. The ground material's fineness is therein defined substantially by the shape of the individual ore fragments, in particular by their diameter or, as the case may be, the spectrum of their size distribution. On the other hand the sensor system needs to be sufficiently robust to operate reliably in the extremely adverse environment because extremely high downtime costs will result from an outage of such a system.
Methods for determining the fineness are known in the case of which the acoustic spectrum or what is termed the “acoustic fingerprint” of the mill's drum during the grinding process is determined. Conclusions about the shape of the ground material can be drawn from the spectrum so that, based on the spectrum's evaluation, a decision can be made as to whether the required fineness has been attained. Proven techniques for determining the spectrum are
measuring by means of acceleration sensors secured directly to the drum, and
using microphones directed at places on the drum's outer skin that emit particularly characteristic frequency data.
However, measuring the acoustic spectrum with the aid of acceleration sensors poses the problem of how to convey the signals from the drum site to a central unit that evaluates the measured signals. That can scarcely be done with adequate reliability by applying classical electrical solutions, for example using ring grinders. While a cableless sensor system such as, for instance, the industrial variant of the WLAN protocol would have better realization prospects, the problem with that is how to make the necessary electric energy available to the system robustly and on a permanent basis.
When the acoustic fingerprint is recorded via suitably positioned microphones, conveying the structure-borne sound from the drum to the microphones via the air constitutes a loss path which may in some circumstances seriously falsify important acoustic information or, as the case may be, not convey it with sufficient quality. In the extremely dusty and otherwise dirty environment it is furthermore doubtful whether microphones are able to meet the requirements placed on stability and robustness.